1. Field of the Invention
The present invention relates generally to nuclear fuel rod tube manufacture and, more particularly, is concerned with an apparatus and method for flush etching, rinsing and drying tubes in the tube manufacturing system.
2. Description of the Prior Art
Because of their critical roles in nuclear reactors, tubes used in fuel rods must meet very stringent quality control standards. Therefore, after formation by multiple pilgering passes, the tubes are subjected to a variety of processing steps which include etching, rinsing and drying thereof.
In one prior art approach, tube etching, rinsing and drying were carried out on a batch basis meaning that the tubes in lot size bundles, usually six hundred tubes to a bundle (or, in some instances, a half lot size bundle of three hundred tubes) were successively etched, rinsed and dried. More particularly, the bundle of tubes are loaded into a carrier and transported by a crane to an etching station where it is dipped into an open-top etching tank. Since the opposite ends of the tubes are open, both the inside and outside diameter surfaces of the tubes are etched. To determine whether sufficient material has been removed from the tube surfaces by the etching, the entire bundle of tubes must be withdrawn from the etching tank one or more times, inspected by an operator and then returned to the tank after each withdrawal if more etching is required. Each withdrawal of the bundle from the etching tank exposes the tubes to atmospheric oxygen.
After completion of tube etching, rinsing of the tubes is carried out in a similar manner. The crane transfers the carrier loaded with etched tubes to a rinsing station. There, the bundle of tubes and carrier are lowered into a water tank and the etched tubes rinsed to neutralize the acid bath. The rinsed bundle of tubes is then removed from the water tank and next hosed down while suspended over a drain. The carrier loaded with the rinsed bundle of tubes is then transferred to a drying station where it is lowered into a forced air chamber. The chamber is closed and a blower and heaters are turned on to force hot air over the bundle to dry the same.
In recent times, to improve manufacturing productivity and quality much emphasis has been placed on automation of fuel rod tube manufacture. However, certain aspects of the steps in the above-described prior art approach have tended to constrain such improvements. One aspect is the large number of tubes in each bundle. In processing such a great quantity of tubes concurrently through etching, rinsing and drying, it is cumbersome and time-consuming to have to transport the bundle from one station to the next, and it is difficult to precisely control the quality of individual tubes without substantial manual handling and tube rework being required. Another aspect is etching of the tubes in the presence of atmospheric oxygen. This step has created the problem of fluoride deposits which makes it necessary to introduce extra steps just to rectify this problem.
In another prior art approach intended to reduce some of the above-mentioned problems, tubes are flush etched by pumping etching acid only through the inside of the tubes individually. A problem with this approach, however, is the difficulty in achieving effective and uniform etching of the tube. For effective and uniform etching, the acid flow and the residence time of the acid in the tube is critical. Ideally, the acid should uniformly contact each portion of the tube inside for the same amount of time and with the same concentration of acid. In actuality, the interaction with the tube metal acts to dilute the acid which then causes a slightly tapered inside diameter surface along the length of the tube. This is caused because the acid is most effective (highest in concentration) at the beginning of a tube, removing the most metal there, and less effective (lowest in concentration) at the end of the tube, removing the least metal there.
One proposed solution to this problem of achieving effective and uniform tube etching is to introduce acid flow through the tube from one end thereof for a specified time and then turn the tube around and introduce flow through the tube from the opposite end. Not only is this proposed solution inefficient, but the residual acid has to be rinse flushed from the tube interior between acid flushes. A further disadvantage of this proposed solution is that it allowed exposure of the acid to the atmosphere, causing fume control problems, acid splash prevention concerns and formation of fluoride deposits in the tubes.
Consequently, a need has evolved for a different approach to tube etching and rinsing in fuel rod tube manufacture which promises improved etching effectiveness and uniformity and thereby improved product quality and reliability.